Put down that low-fat yogurt! Dr. Robert Lustig says that sugar - not fat - may be the real culprit behind America's obesity epidemic.

In the 1970s, America found itself embroiled in a full-on war: on fats, that is. Low-fat foods became all the rage, and supermarket shelves stocked up on a smorgasbord of low-fat yogurts, pastries, you name it. But, in the name of going low-fat, food manufacturing companies began putting their products through quite a transformation.

"When you take the fat out of the food, it tastes like cardboard. It tastes nasty!" Lustig says. "The food industry knew that. They said, 'What are we going to do to make the food palatable?' The answer: add the sugar."

Adding sugar proved to be an incredibly lucrative move. Now, of the 600,000 items in the American food supply, 80% are spiked with added sugar. Today, Lustig points out that as companies pumped sugar into their products - and American diets - obesity rates rose. In 1960, fewer than 15% of Americans were obese. That percentage has now just about tripled.

As a result, scientists have gone back into the lab and tried to figure out how sugar affects the body - and not just for those who are overweight. Lustig argues that 40% of the normal-weight population have the same metabolic dysfunctions that the overweight do, which leads to conditions like diabetes and heart disease.

"The reason is because it's not about obesity, it's about the inflammation that comes with what we eat that leads to the metabolic dysfunction and even normal weight people can get it," Lustig says. "Obesity is a red herring"

For more on Lustig's research - including how sugar can affect not only our physical health, but our mental health - tune in to our full interview, above.

Still curious?

Hear our interview with Dr. David Ludwig on the addictive properties of carbs.